In recent years, the ink for ink jet printing process has been required to give a printed image excellent in fastness such as water resistance and light-resistance and be free from disadvantages, e.g., irregular ink flow and expansion of ink droplets attached to the printing medium (hereinafter referred to as “blurring”) regardless of the kind of printing medium.
To this end, it has been considered to use a pigment having an excellent fastness instead of dye for the purpose of providing printed images with desired fastness. Unlike dyes, pigments cannot be dissolved in water. Thus, it is necessary that a pigment be finely dispersed in water. However, it is very difficult to keep the pigment dispersed in water in a stable manner. In order to solve this problem, various techniques for dispersing a pigment in water in a stable manner have been proposed. As one of these techniques there is known a technique involving the incorporation of sulfonic acid groups into the surface of pigment particles.
For example, Japanese Patent Laid-Open No. 1998-110129 discloses an ink for ink jet printing process comprising a “sulfonated surface-treated organic pigment” obtained by treating an organic pigment dispersed in a solvent free of active proton with a sulfonating agent. In accordance with this approach, the ink for ink jet printing process has a stable dispersibility and thus can be ejected from the nozzle in a stable manner. Further, Japanese Patent Laid-Open No. 1999-49974 discloses that an organic pigment mass having a sulfonic acid group incorporated therein is treated with a monovalent metal ion to prepare an organic pigment mass which can be positively charged on the surface thereof. This document also discloses an aqueous ink composition having an excellent storage stability comprising a pigment emulsion prepared from the positively surface-charged organic pigment mass, a dispersant and water.
The aforementioned related art techniques have the following disadvantages.
Specifically, the pigment having a sulfonic acid group incorporated therein disclosed in the above cited Japanese Patent Laid-Open No. 1998-110129 and Japanese Patent Laid-Open No. 1999-49974 is free of fixing component on the surface thereof and thus is disadvantageous in that the resulting printed image exhibits deteriorated friction resistance. Thus, Japanese Patent Laid-Open No. 1998-110129 discloses that the fixability of the printed image can be improved by the addition of a water-soluble resin.
However, the ink having the formulation disclosed in Japanese Patent Laid-Open No. 1998-110129 exhibits a poor penetrating power when printed on a special printing medium (e.g., glossy paper, glossy film sheet) which is used for the case where an image having a quality as high as or higher than color photograph is required. As a result, the liquid component in the ink penetrates little into the printing medium, leaving the pigment particles behind on the surface of the printing medium. Thus, the resulting printed image loses surface smoothness and lacks gloss, impairing the print quality and friction resistance. Further, ink blurring occurs on the area where two or more color inks are imposed on each other or come in contact with each other, particularly upon full-color printing.